J Schanda

1
Characterizing illumination systemsColour
rendering and beyond

• J Schanda
• University Veszprém, Department of Image
  Processing and Neurocomputing, Hungary

2
CIE Standards for assessing quality of light
sources

• Introduction
• Characterisation of the light source
• Colour rendering
• Assessing daylight simulators
• CIE Technical Committees working on the above
  questions
• New approaches
• Summary

3
CIE Standards and Guides

• CIE colorimetric illuminants
• ISO/CIE 10526, CIE S005
• CIE colorimetric observers
• ISO/CIE 10527
• International Lighting Vocabulary
• CIE 17.4
• Colorimetry CIE 15.2, new edition coming
• Colour rendering CIE 13.3, outdated -
  experiments needed
• Quality of daylight simulators CIE 51, possible
  improvements

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Illuminant, source, simulator

• CIE Standard Illuminant
• Defined as spectral distribution

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CIE Standard Illuminants
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Illuminant, source, simulator

• CIE Standard Illuminant
• Defined as spectral distribution
• CIE Standard Source
• Source with specified spectral distribution
• Simulator
• Source approximating the illuminant

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CIE colorimetric illuminants

• CIE Standard Illuminant A
• New definition, unchanged values

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CIE Standard Illuminant A
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CIE colorimetric illuminants

• CIE Standard Illuminant A
• New definition, unchanged values
• CIE Standard Illuminant D65

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CIE Standard Illuminant D65
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CIE colorimetric illuminants

• CIE Standard Illuminant A
• New definition, unchanged values
• CIE Standard Illuminant D65
• Other daylight illuminants D50
• Evaluation

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CIE (Standard) Source(s)

• CIE Standard Source A
• Realization incandescent lamp
• D65 no standard source
• Secondary sources
• fluorescent lamps

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Source standards and imaging

• Original
• Real life daylight illumination
• paper copy D50, fluorescent or tungsten illum.
• screen representation
• Reproduction
• screen representation
• hard-copy

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Source standards and imaging

• Original scene
• Daylight D50, D65 (Jackson - MacDonald - Freeman
  pictures)

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Source standards and imaging

• Reproduction
• screen representation
• CRT primaries
• chromatic adaptation
• colour appearance models
• hard-copy
• CMYK primaries
• base material (paper reflection properties)
• gamut mapping

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Source standards original scene

• Light source colour description
• chromaticity of source
• colour temperature
• correlated colour temperature
• metamerism index
• Colour rendering
• special indices Ri
• general colour rendering index Ra

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Light source colour description

• Colour temperature, Tc
• The temperature of a Planckian radiator whose
  radiation has the same chromaticity as that of a
  given stimulus.
• Correlated colour temperature Tcp
• The temperature of the Planckian radiator whose
  perceived colour most closely resembles that of a
  given stimulus at the same brightness and under
  specified viewing conditions.

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Light source colour description
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Colour rendering

• Spectra of two sources of equal chromaticity

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Colour rendering

• Chromaticity of the two sources and of a sample
  illuminated by these sources

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Colour rendering

• Effect of an illuminant on the colour appearance
  of objects by conscious or subconscious
  comparison with their colour appearance under a
  reference illuminant

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Colour rendering index

• Measure of the degree to which the
  phsychophysical colour of an object illuminated
  by the test illuminant conforms to that of the
  same object illuminated by the reference
  illuminant, suitable allowance having been made
  for the state of chromatic adaptation.

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Colour rendering index

• Comparison of illuminants
• test illuminant
• reference illuminant
• Planckian if CCT lt 5000 K
• Phase of Daylight ? 5000 K
• Objects illuminated (test colour samples)
• Chromatic adaptation
• Colour difference

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Flow-chart of CRI calculation
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Test colour samples
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CRI calculation - 1

• Selecting reference illuminant
• Selecting test colour samples
• Chromatic adaptation (von Kries)
• Colour difference calculation
• U, V, W colour space

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Chromatic adaptation
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CRI calculation - 2

• Special colour rendering indices Rk,t
  100 4,6 ? Ei,k,r-t.
• General colour rendering index

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Assessing daylight simulators - 1

• For colorimetric practice the evaluation of the
  UV content also important
• Assessment based on metamerism indices
• 5 metameric pairs
• 3 sets D55, D65, D75,
• Set for D50 under development

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Assessing daylight simulators - 2

• Visible range metamerism index
• average colour difference for the five metameric
  pairs

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Assessing daylight simulators - 3

• UV range assessment
• reflected part of radiation
• emitted part of radiation
• Final assessment MI for UV and visible
• Assessment based on CIELAB or CIELUV colour
  differences
• Five categories established

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Assessing daylight simulators - 4
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CIE TCs working on colorimetry

• CIECAM colour appearance models
• VDU - Reflective media comparison
• Chromaticity diagram with physiologically
  significant axes
• Geometric tolerances in colorimetry
• Updating the colorimetry and colour rendering
  documents

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Summary

• Light source colorimetry
• lamp light colour
• brightness - task-performance relationship
• colour rendering
• Image reproduction colorimetry
• colour appearance
• colour management